spi: spi-geni-qcom: Fix the clk div computation.

A bracket error could leave us with a clk divider value of 0 for the
Serial Engine which will affect the GENI sequencer.

Change-Id: I2dd086b936b6f60957bf6bbefe8a3cdd831cf3d7
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>